1. Field of the Invention
The present invention relates to technology for forming images.
2. Description of the Related Art
There have been proposed and developed image forming apparatuses of an electrophotographic type and an ink-jet type, which form finer images using toners or inks of five or more colors. Japanese Patent Application Laid-Open Nos. 2002-31949 and H8-171252 disclose such image forming apparatuses. These image forming apparatuses use a dark toner and a light toner to improve the granularity at a portion in the image where the density is low.
Some other publications teach forming images using a dark ink and a light ink to improve the granularity at a portion in the image where the density is low. Such a technology has been disclosed in Japanese Patent Publication No. 3292104.
Such image forming apparatuses record data in a binary form of dots being ON/OFF or in several gray scales using dots of an intermediate size. One approach to perform such a recording is to perform halftone processing on each piece of color data of the input image data to convert the number of gray scales of input image data to the number of recording gray scales of the image forming apparatus. Dithering is one of the types of the halftone processing. When dithering is performed, an image having a periodicity, such as a halftone screen or a line screen, is formed. The halftone screen expresses darkness or lightness by the size of a halftone, while the line screen expresses darkness or lightness by the width of a line.
An image forming apparatus that records data in four colors of cyan, magenta, yellow, and black (CMYK) generally uses different screens for individual colors to suppress a moire generated by periodical overlapping of individual colors. To suppress a moire originated from overlapping of halftone screens of three planes, it is generally desirable to lay out the screens at angles of 30 degree when the frequencies of halftones (the numbers of lines) are the same. Three planes of CMK are often laid out at intervals of 30 degree, with the Y plane, hardest to be observed, being laid out at an intermediate screen angle. For a line screen, it is possible to suppress a moire by arranging the three CMK planes at intervals of 60 degree.
Since the screen angle and the frequency that can be taken are generally determined by the recording resolution of an image forming apparatus, the screen setting that is closest to the above condition is chosen. When a halftone screen with the recording resolution of 600 dots per inch (dpi) or 1200 dpi is used, for example, the setting is mainly as follows.                C plane 190L (18 degrees)        M plane 190L (72 degrees)        Y plane 200L (0 degree)        K plane 212L (45 degrees).        
FIGS. 2A and 2B are explanatory diagrams of a halftone screen 51 and a line screen 52 with a screen angle θ of 18 degrees according to the conventional technique. As there are two screen angles between 0 degree and 180 degree for a halftone screen, the screen angle is expressed by an angle of 0 degree or greater and less than 90 degree in the angle range, whereas the screen angle is expressed by an angle of 0 degree or greater and less than 180 degree for a line screen.
However, when forming an image with a dark toner and a light toner (or dark ink and light ink) of five colors for the K plane (hereinafter, the dark K plane is referred to as “Bk plane” while the light K plane is referred to as “Lk plane”), it is necessary to consider a moire phenomenon generated by overlapping of the screens of the Bk plane and the Lk plane, and also a moire generated by the Bk plane and another color plane and a moire generated by the Lk plane and another color plane.
When, for example, the CMYK planes are set to CMYBk planes and the halftone screen of the Lk plane is arranged between the C plane and the Bk plane, for 1200 dpi, a halftone screen of 206L (31 degrees) can be set. Here, the screen frequencies of the Bk plane and the Lk plane are almost the same and there is an angle difference of 14 degrees; therefore a moire is generated when the two planes overlap each other. That is, for each of the individual planes of five colors, it is necessary to determine the desirable screen setting to prevent a moire generated by overlapping of individual planes.
FIG. 3 is an explanatory diagram of an example where a moire is generated by overlapping of planes for the screen for the Bk plane set to 212L (45 degrees) and the screen for the Lk plane set to 206L (31 degrees). With the setting as shown in FIG. 3, overlapping of the Bk plane and the Lk plane generates a moire on the screen.